The electrophotographic developing method includes sticking of the toner particles in a developer on an electrostatic latent image formed on a photo conductor. The developers used in the method can be divided into two groups: a binary-component developer composed of toner particles and carrier particles, and a single-component developer using toner particles only. As the carrier particles, ferrite carrier particles composed of ferrite particles are used.
As the developing method using a binary-component developer composed of toner particles and carrier particles, a cascade method was used in old times and a magnetic brush method using a magnet roll is mainly used in the present days.
In a binary-component developer, the ferrite carrier particles are a support substance which gives a desired electric charge on a toner particle in stirring together with toner particles, and carries the toner particles with the electric charge to the surface of a photo conductor to form a toner image on the photo conductor. The ferrite carrier particles remaining on a developing roll holding a magnet return into a developing box again from the developing roll for mixing and stirring with new toner particles for repeated use in a certain period.
Different from in a single-component developer, a binary-component developer has a good controllability in designing a developer because ferrite carrier particles mixed and stirred with toner particles achieve functions for triboelectrically charging and carrying toner particles. So, a binary-component developer is suitable used in a full-color developing device required high definition image quality and a high-speed printer required reliability and durability of an image.
In the binary-component developer used in such a manner, image qualities such as image density, fogging, white spots, tone reproduction and resolution should be a specific level in the early stage and these qualities should be stably achieved without fluctuation in an endurance printing. To stably achieve the qualities, the ferrite carrier particles contained in the binary-component developer should have stable properties.
Patent Document 1 (Japanese Patent Laid-Open 2012-230373) discloses a resin carrier coated with a resin constituted by spherical composite particles composed of at least fine particles of ferromagnetic iron oxide as a magnet carrier core material and a phenol resin.
Patent Document 2 (Japanese Patent Laid-Open 2012-58344) discloses a ferrite carrier core material composed of porous ferrite particles that have the composition represented by (MnO)x(MgO)y(Fe2O3)z, and a part of (MnO)(MgO) and/or (Fe2O3) is replaced by 0.3 to 4.0 wt % of SrO. Further, a resin filled ferrite carrier used for an electrophotographic developer wherein the pores of the ferrite carrier core material are filled with resin is disclosed.
Patent Document 3 (Japanese Patent Laid-Open No. 2007-34249) discloses a method of manufacturing a ferrite carrier used for an electrophotographic developer including the steps of preparing a granulated material by mixing resin particles, a binder, a dispersant, a moistening agent water and a raw material powder, wet pulverizing the mixture and drying the pulverized material, preparing the porous carrier core material by calcining and firing the granulated material and applying a resin on the porous carrier core material, and finish a resin coated ferrite carrier.
In recent years, the amount of carrier used tends to increase due to employment of the trickle development. On the other hand, the amount of developer and carrier used has been attempted to reduce for running cost reduction of a developer.
To reduce the amount of carrier used, a carrier having a low apparent density has been proposed. If the developing machines have the same volume, lower apparent density of carrier can reduce the weight used.
If a resin coated carrier is used for a long time, the coated resin may be peeled off due to the internal stress of a developing machine and the charging properties and the electric resistance required for image formation may change to make achievement of the stable images for a long time difficult.
From the viewpoint of energy saving, low-temperature fixing of toner has become widespread. However, negative effect on the image is worried because the low-temperature fixing of toner makes toner easily stick to the surface of a carrier due to the internal stress of a developing machine and fluctuation in charging properties of the carrier.
Although a carrier filled with resin and a resin carrier have been investigated to solve these problems, the large amount of resin used may result high electric resistivity and it causes difficulty in electric resistivity control of the carrier. Furthermore, as the charging properties are greatly influenced by the resin, available conditions of the carriers might be limited.
To solve these problems, a ferrite particle as a carrier not only low in specific weight but also easy in controlling of the charging properties and electric resistivity is required.
Patent Document 4 (Japanese Patent Laid-Open No. 2007-320847) discloses products containing a plurality of primary fine particles and core fine particle structures containing a plurality of primary pores and core-shell ceramic fine particles containing a shell surrounding at least a part of core fine particle structure. The products include a membrane, a sensor, an electrode, and a getter.
The core-shell ceramic fine particles disclosed in Patent Document 4 is constituted by yttrium stabilized zirconia and lanthanum ferrite as the shell. As lanthanum ferrite is used as the shell, the ferrite particles are low in apparent density and filling of a specified volume with a low weight with various properties maintained in a controllable state may not be achieved.